A power circuit for use in a TV receiver having a satellite broadcast receiving function or a teletext receiving function frequently comprises:
a main power source for supplying a large power to the main load, i.e., the signal-reception, signal-processing, and display circuits of the TV receiver (hereinafter, called the TV circuit); and PA1 a sub-power source supplying power to small-power drawing circuits such as a satellite broadcast receiving tuner, a UHF/VHF (U/V) tuner, a teletext receiving circuit, a microprocessor, and a remote control receiving circuit. PA1 a transformer including a secondary winding and a tertiary winding, magnetically coupled with a primary winding, and a switching element; PA1 a switch circuit connected in series with one end of the secondary winding for supplying power to a TV circuit and a first rectifier-and-smoothing circuit connected in series with the other end of the switch circuit; PA1 a second rectifier-and-smoothing circuit connected with the tertiary winding for supplying power to a different circuit from the TV circuit; PA1 a first output-voltage-detection and error-amplifier circuit for obtaining signal corresponding to the difference between a detected value of a first output voltage obtained from the first rectifier-and-smoothing circuit and a first reference voltage; PA1 a second output-voltage-detection and error-amplifier circuit for obtaining a signal corresponding to the difference between a detected value of a second output voltage obtained from the second rectifier-and-smoothing circuit and a second reference voltage; PA1 a microprocessor for activating the switch circuit and selectively activating the first and the second output-voltage-detection and error-amplifier circuits; PA1 a control circuit including a light-emitting element, responding commonly to the output of the first or second output-voltage-detection and error-amplifier circuit, and a photo-receiving element optically coupled with the light-emitting element for controlling the switching element such that the first or second output voltage is controlled to be constant in accordance with the change in the resistance value of the photo-receiving element; and PA1 a circuit for temporarily raising the output voltage obtained from the second rectifier-and-smoothing circuit immediately before the switch circuit is turned ON.
FIG. 5 shows a circuit of the switching power source of a TV receiver having such a sub-power source as described above. In FIG. 5, AC source terminal 1 is connected to rectifier circuit 3 for converting AC to DC power source through power switch 2. A pair of DC output terminals 4 and 5 of rectifier circuit 3 have transistor 8 as the switching element connected therebetween through primary winding 7 of transformer 6.
Transformer 6 has secondary winding 9, tertiary winding 10, and fourth winding 11 magnetically coupled with primary winding 7. Fourth winding 11, connected between the base and emitter of transistor 8 through a parallel circuit of resistor 12 and capacitor 13, is a drive winding allowing transistor 8 to turn ON/OFF in a self-excited manner by means of positive feedback. Starting resistor 14 is connected between DC terminal 4 of rectifier circuit 3 and the base of transistor 8. Phototransistor 16, connected between the base and emitter of transistor 8, is a constant-voltage control element. Phototransistor 16 serves as a bypass for allowing the base current of transistor 8 to pass therethrough so that constant-voltage control of the output voltage of transistor 8 is achieved.
Secondary winding 9 is connected with first rectifier-and-smoothing circuit 19 formed of first rectifier diode 17 and first smoothing capacitor 18. The input line to first smoothing diode 17 is connected with switch 20. Output line 21 of first rectifier-and-smoothing circuit 19 is connected to TV circuit 22 as the main load.
Tertiary winding 10 is connected to second rectifier-and-smoothing circuit 25 formed of second rectifier diode 23 and second smoothing capacitor 24. Output line 26 of second rectifier-and-smoothing circuit 25 is connected to satellite tuner 27, U/V tuner 28, microprocessor 50, and remote control receiving circuit 29 as the sub-load.
To detect the voltage on output line 26 for achieving constant-voltage control while power is being supplied only to the sub-load, second output line 26 is connected with second output-voltage-detection and error-amplifier circuit 39, which is formed of voltage detecting resistor 41, error amplifying transistor 42, and zener diode 43 as a second reference voltage source. Resistor 41 is connected between second output line 26 and the base of transistor 42.
Zener diode 43 is inserted between the collector of transistor 42 and the cathode of light emitting diode 37, with its anode connected to the collector of transistor 42 and its cathode connected to the cathode of light emitting diode 37. The anode of light emitting diode 37 is connected to second output line 26 through a resistor 36. Light emitting diode 37 is in common use for first and second output-voltage-detection and error-amplifier circuits 30 and 39.
Switch circuit 40 is constituted of transistor 45. Transistor 45 has its collector connected to the base of second error amplifying transistor 42 and its emitter connected to the ground.
When power is supplied to TV circuit 22 as the main load, microprocessor 50 applies a high-level output of TV-circuit-ON/OFF-control signal 46 to the base of transistor 45 of switch circuit 40. Thereby, while switch 20 is turned ON, transistor 45 of output switch circuit 40 is turned ON and second error amplifying transistor 42 is turned OFF.
Upon turning ON of switch 20, the voltage on output line 21 from first-rectifier-and-smoothing circuit 19 rises. To detect the voltage on output line 21 and perform constant-voltage control, there is output-voltage-detection and error-amplifier circuit 30 connected between first output line 21 and the ground. Circuit 30 is made up of resistors 31 and 32 connected between output line 21 and the ground, error amplifying transistor 33, zener diode 34 as a reference voltage source, and a resistor 35.
The base of transistor 33 is connected to the voltage dividing point of resistors 31 and 32. Zener diode 34 is connected to the emitter of transistor 33. Resistor 35 is inserted between output line 21 and the cathode of zener diode 34. Accordingly, a collector current corresponding to the difference between the voltage on output line 21 and the reference voltage given by zener diode 34 is allowed to flow. At this time, second output-voltage-detection and error-amplifier circuit 39 is inoperative because error amplifying transistor 42 is OFF.
Light emitting diode 37, connected between output line 21 and the collector of transistor 33 through resistor 36, emits a light quantity corresponding to the flow of collector current of transistor 33 to provide an optical input to phototransistor 16 optically coupled with light emitting diode 37.
Operations of transistor 8 as the switching element and the energy transferring operation performed through transformer 6 will be described.
When power switch 2 is turned ON, a flow of base current is produced in transistor 8 through starting resistor 14 and hence transistor 8 is turned ON. Upon turning ON of transistor 8, a positive feedback voltage is produced in fourth winding 11 so that transistor 8 is held ON. Since primary winding 7 of transformer 6 has an inductance, the collector current of transistor 8 increases with time.
The largest value of the collector current of transistor 8 is determined by the base current and the current amplification factor. When the collector current reaches the value equivalent to the product of the base current and the current amplification factor, transistor 8 is turned into its unsaturated state so that the positive feedback voltage in fourth winding 11 decreases. As a result, transistor 8 is turned into its OFF state. First and second rectifier diodes 17 and 23 are held OFF while transistor 8 is ON, and held ON while transistor 8 is OFF.
Upon turning OFF of transistor 8, the energy stored in the inductance of transistor 6 causes a flyback voltage to be generated. When switch 20 is OFF so that power is supplied only to the sub-load, only second smoothing capacitor 24 is charged with the generated flyback voltage. When switch 20 is ON so that power is also supplied to the TV circuit as the main load, both first and second smoothing capacitors 18 and 24 are charged.
When the main load, i.e., TV circuit 22, is started to operate, switch 20 and switch circuit 40 are turned ON with the sub-load, i.e., satellite tuner 27, U/V tuner 28, and the like, held operative. In this state, switch 20 and switch circuit 40 are turned ON by a signal from microprocessor 50. At this time, the main load drawing a much greater power than that drawn by the sub-load is suddenly put into connection with output line 21. Therefore, a phenomenon of voltage drop as shown in FIG. 6 occurs on output line 26.